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Abstract:

A crystal material lattice strain evaluation method includes illuminating
a sample having a crystal structure with an electron beam in a zone axis
direction, and selectively detecting a certain diffracted wave diffracted
in a certain direction among a plurality of diffracted waves diffracted
by the sample. The method further includes repeating the illuminating
step and the selectively detecting step while scanning the sample, and
obtaining a strain distribution image in a direction corresponding to the
certain diffracted wave from diffraction intensity at each point of the
sample.

Claims:

1. A method of evaluating a distribution of lattice strain on crystal
material, the method comprising: illuminating a sample having a crystal
structure with an electron beam in a zone axis direction (termed as
"illuminating step") and selectively detecting a certain diffracted wave
diffracted in a certain direction among a plurality of diffracted waves
diffracted by the sample (termed as "selectively detecting step"); and
repeating said illuminating step and said selectively detecting step
while scanning the sample, and obtaining a strain distribution image in a
direction corresponding to the certain diffracted wave from diffraction
intensity at each point of the sample (termed as "strain distribution
image obtaining step").

2. The method according to claim 1, further comprising steps of:
repeating said strain distribution image obtaining step for each of a
plurality of diffracted waves diffracted in directions different from
each other and obtaining strain distribution images in directions
corresponding to the directions of the diffracted waves, based on the
diffracted waves diffracted in directions different from each other; and
executing stress analysis on the sample, based on the strain distribution
images in the plurality of directions.

3. The method according to claim 2, further comprising: quantifying the
magnitude of strain of the strain distribution image.

4. The method according to claim 3; wherein executing the stress analysis
further comprises: calculating shear strain in a certain direction, based
on strain measurement results in a plurality of directions; and
calculating the magnitude and direction of principal strain, based on
strain measurement results in a plurality of directions and the
calculated shear strain in a certain direction.

5. The method according to claim 3; wherein executing the stress analysis
further comprises: calculating shear strain in a certain direction, based
on strain measurement results in a plurality of directions; and obtaining
shear strain in an arbitrary direction, based on the strain measurement
results in a plurality of directions and the calculated shear strain in a
certain directions.

6. The method according to claim 1; wherein the sample is a TEM sample
which is formed by thinning material having a crystal structure to have a
uniform thickness, and the certain diffracted wave is obtained by forward
scattering.

7. The method according to claim 1; wherein, in said selectively
detecting step, the electron beam with which the sample is illuminated is
narrowed to a rectangle having long and short sides perpendicular and
parallel to a direction in which the certain diffracted wave is
diffracted with respect to the transmitted wave, respectively, and the
narrowed electron beam is focused on the sample.

8. A system of evaluating a distribution of lattice strain on crystal
material, the system comprising: a scanning transmission electron
microscope that illuminates a sample with an electron beam, scans the
sample, and detects a diffracted wave transmitted or diffracted by the
sample; and a strain distribution image extraction unit that selects a
certain diffracted wave among diffracted waves transmitted or diffracted
by the sample, and obtains a strain distribution image.

9. The system according to claim 8, further comprising: a strain
quantification unit that quantifies strain intensity of the strain
distribution image.

10. The system according to claim 8, further comprising: a stress
analysis unit; wherein the strain distribution image extraction unit
obtains strain distribution images in a plurality of directions each
corresponding to the direction of one of a plurality of diffracted waves
diffracted in directions different from each other; and wherein the
stress analysis unit executes stress analysis on the sample, based on the
strain distribution images in a plurality of direction obtained by the
strain distribution image extraction unit.

11. The system according to claim 10; wherein the stress analysis unit is
configured to execute processes of: calculating shear strain in a certain
direction, based on the strain distribution images in a plurality of
directions; and calculating the magnitude and direction of principal
strain, based on the strain distribution images in a plurality of
directions and the calculated shear strain in a certain direction.

12. The system according to claim 10; wherein the stress analysis unit is
configured to execute processes of: calculating shear strain in a certain
direction, based on the strain distribution images in a plurality of
directions; and calculating shear strain in an arbitrary direction, based
on the strain distribution images in a plurality of directions and the
calculated shear strain in a certain direction.

13. The system according to claim 8; wherein the scanning transmission
electron microscope further comprises a sample orientation control
apparatus that adjusts crystal orientation of the sample with respect to
the direction of the incident electron beam.

14. The system according to claim 8, further comprising: a display
apparatus that displays evaluation results including the strain
distribution image.

15. The system according to claim 8; wherein the scanning transmission
electron microscope comprises an illumination aperture having a
rectangular opening to narrow the electron beam and illuminate the sample
with the narrowed electron beam; and wherein the rectangular opening of
the illumination aperture can be set so that the long and short sides
thereof are perpendicular and parallel to a direction in which the
certain diffracted wave is diffracted with respect to the transmitted
wave, respectively.

16. The system according to claim 15; wherein the illumination aperture
comprises a plurality of rectangular openings, each of which has long and
short sides in different directions; and wherein, in accordance with a
direction in which the certain diffracted wave is diffracted, an opening
is selectable from the plurality of openings as an aperture to illuminate
the sample with the electron beam with the selected opening.

17. A non-transitory computer-readable recording medium storing a
computer program used in an evaluation system comprising a scanning
transmission electron microscope and a computer that controls the
scanning transmission electron microscope and processes measurement data
obtained by the scanning transmission electron microscope, the computer
program causing the computer to execute processes of: controlling the
scanning transmission electron microscope so that a sample having a
crystal structure is illuminated with an electron beam and a diffracted
wave transmitted or diffracted by the sample is detected; and scanning
the sample, selecting a certain diffracted wave among diffracted waves
transmitted or diffracted by the sample, and obtaining a strain
distribution image.

18. The non-transitory computer-readable recording medium according to
claim 17; wherein said scanning transmission electron microscope
comprises an illumination aperture having a rectangular opening whose
direction is changeable; and wherein said process of controlling the
scanning transmission electron microscope includes controlling of the
direction of the rectangular opening of the illumination aperture in
accordance with a direction in which the certain diffracted wave is
diffracted with respect to the transmitted light.

Description:

TECHNICAL FIELD

REFERENCE TO RELATED APPLICATION

[0001] This application is based upon and claims the benefit of the
priorities of Japanese patent application No. 2011-112618, filed on May
19, 2011 and Japanese patent application No. 2012-034676, filed on Feb.
21, 2012 the disclosure of which is incorporated herein in its entirety
by reference thereto.

[0002] The present invention relates to a method and a system of
evaluating a distribution of lattice strain on crystal material. In
particular, it relates to a method and a system of evaluating a
distribution of lattice strain on crystal material used in a
semiconductor device or the like by using electron beam diffraction.

BACKGROUND

DESCRIPTION OF THE RELATED ART

[0003] When an LSI device is manufactured, stress generated by use of
various types of material causes lattice strain on a crystal structure
used in a semiconductor device. Such lattice strain is one of the
important physical quantities that exhibit crystal material properties.
The stress and lattice strain change, depending on the difference in
mechanical physical properties of various types of material used in LSI
device processes or depending on heat treatment used in processes. The
lattice strain is a cause of a crystal defect or the like, resulting in
device failure. In addition, in recent years, for example, attempts are
being made to improve the electron and hole mobility of silicon, by using
the lattice strain. Namely, attempts are being made to improve physical
properties of crystal material by actively utilizing the lattice strain.
Thus, if the lattice strain is utilized properly, improvement in device
performance can be expected. However, if the lattice strain is not
controlled properly, a crystal defect leading to device malfunction is
caused. Therefore, evaluation of the stress and lattice strain on crystal
material and optimization of process conditions are essential in the
development of LSI devices.

[0004] Conventionally, the stress and lattice strain on crystal material
such as in a semiconductor device have been evaluated by X-ray
diffractometry, Raman spectroscopy, or the like. However, recent
reduction in device size is making these conventional stress and lattice
strain evaluation methods insufficient in spatial resolution, and it is
becoming more difficult to obtain sufficient results. Therefore, a
convergent-beam electron diffraction (CBED) method and a nano-beam
electron diffraction (NBD) method are being proposed as methods using an
electron beam and evaluating localized stress and lattice strain.

[0005] For example, Patent Document 1 discloses an apparatus and a method
using the CBED method and evaluating strain on crystal material. In
addition, Non-Patent Document 1 discloses using the NBD method and
evaluating strain on an SOI MOSFET. In addition, Patent Documents 2 and 3
disclose a method of using a diffraction contrast and two-dimensionally
evaluating lattice strain instantly. Non-Patent Document 2 discloses a
strain evaluation method using thermal diffuse scattering electron
intensity. Patent Document 4 discloses a semiconductor device having a
trench-gate transistor as an example of a minute device structure
requiring localized stress and lattice strain evaluation.

[0018] The disclosure of the above Patent Documents and Non-patent
Documents are incorporated herein in their entirety by reference thereto.
The following analysis is given by the present invention. Since the above
CBED and NBD methods are used for evaluation on one-dimensional points,
it is insufficient to discuss the influence caused by lattice strain on
device characteristics. Even with such evaluation method based on the
CBED and NBD methods, in principle, it is possible to set many evaluation
points and obtain a two-dimensional distribution. However, since the
analysis method requires complicated and time-consuming operations, such
method is not effective. In addition, these evaluation methods are not
established as evaluation apparatuses. The evaluation methods are merely
known as application examples of a transmission electron microscope.
Namely, skilled techniques are required to obtain reliable results.

[0019] In addition, based on the evaluation methods disclosed in Patent
Documents 2 and 3 and Non-Patent Document 2, lattice strain in a variety
of directions is added and measured, it is insufficient to discuss the
influence caused by lattice strain in a current direction. Generally, LSI
devices are planarly formed on a silicon semiconductor wafer or the like
and are designed so that current flows in one direction. Thus, to discuss
the influence caused by strain on device characteristics, it is necessary
to evaluate strain, in view of directional components, such as a strain
distribution, principal strain, and shear lattice strain in each of
various directions.

[0020] According to a first aspect of the present invention, there is
provided a method of evaluating a distribution of lattice strain on
crystal material. The method comprises illuminating a sample having a
crystal structure with an electron beam in a zone axis direction (termed
"illuminating step"), and selectively detecting a certain diffracted wave
diffracted in a certain direction among a plurality of diffracted waves
diffracted by the sample (termed "selectively detecting step"). The
method further comprises repeating the illuminating step and the
selectively detecting step while scanning the sample, and obtaining a
strain distribution image in a direction corresponding to the certain
diffracted wave from diffraction intensity at each point of the sample.

[0021] According to a second aspect of the present invention, there is
provided a system of evaluating a distribution of lattice strain on
crystal material. The system comprises a scanning transmission electron
microscope that illuminates a sample with an electron beam, scans the
sample, and detects a diffracted wave transmitted or diffracted by the
sample. The system further comprises a strain distribution image
extraction unit that selects a certain diffracted wave among diffracted
waves transmitted or diffracted by the sample and obtains a strain
distribution image.

[0022] According to a third aspect of the present invention, there is
provided a non-transitory computer-readable recording medium storing a
computer program used in an evaluation system. The evaluation system
comprises a scanning transmission electron microscope and a computer that
controls the scanning transmission electron microscope and processes
measurement data obtained by the scanning transmission electron
microscope.

[0023] The computer program causes the computer to execute processes of:
controlling the scanning transmission electron microscope so that a
sample having a crystal structure is illuminated with an electron beam
and a diffracted wave transmitted or diffracted by the sample is
detected; and scanning the sample, selecting a certain diffracted wave
among diffracted waves transmitted or diffracted by the sample, and
obtaining a strain distribution image.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a block diagram illustrating an overall configuration of
a system of evaluating a distribution of lattice strain on crystal
material according to an exemplary embodiment of the present disclosure.

[0025]FIG. 2 is a flow chart illustrating a method of evaluating a
distribution of lattice strain on crystal material according to an
exemplary embodiment of the present disclosure.

[0026]FIG. 3A illustrates the direction in which an electron beam is
incident on crystal of a semiconductor device as an evaluation sample and
FIG. 3B illustrates directions of diffracted waves.

[0027] FIGS. 4A and 4B illustrate a principle of obtaining a strain
distribution image from a diffraction image of a certain diffracted wave:
FIG. 4A illustrates a relationship among a reciprocal lattice point, an
Ewald sphere, and an excitation error; and FIG. 4B illustrates the
relationship between a reciprocal lattice point and an excitation error
when lattice strain is caused.

[0028] FIGS. 5A to 5C illustrate a diffracted wave, a diffracted wave
intensity image, and a stress analysis diagram, respectively, used for
analyzing strain in the X direction; and FIGS. 5D to 5F illustrate a
diffracted wave, a diffracted wave intensity image, and a stress analysis
diagram, respectively, used for analyzing strain in the Y direction.

[0029]FIG. 6A illustrates a diffracted wave intensity image; FIG. 6B
illustrates a conversion scale between the diffracted wave intensity and
the lattice strain magnitude; and FIG. 6C illustrates a calibration curve
of the diffracted wave intensity and the lattice strain obtained by the
NBD method.

[0030]FIG. 7 is a flow chart illustrating a method of analyzing lattice
strain from a lattice strain distribution.

[0031]FIG. 8A is a plan view when an STI interface is formed
perpendicular to the crystal axis of an active region; FIG. 8B is an
enlarged view illustrating the stress direction; FIG. 8C is a plan view
when an STI interface is formed with a taper angle with respect to the
crystal axis; and FIG. 8D is an enlarged view illustrating the stress
direction.

[0032] FIGS. 9A and 9B illustrate a method of determining a stress source
from the magnitude and direction of principal strain at each point.

[0033] FIG. 10A illustrates the direction in which stress is caused and
FIG. 10B illustrates the direction in which a reciprocal lattice point is
moved when perpendicular stress is caused on a sample.

[0034]FIG. 11A illustrates the direction in which stress is caused and
FIG. 11B illustrates the direction in which a reciprocal lattice point is
moved when complex stress is caused on a sample.

[0035] FIGS. 12A and 12B illustrate shapes of Ewald spheres formed in a
reciprocal space when the electron beam incident on a sample has a large
convergent angle and a small convergent angle, respectively.

[0036]FIG. 13 is a block diagram illustrating an overall configuration of
a system of evaluating a distribution of lattice strain on crystal
material according to a fourth exemplary embodiment.

[0037] FIG. 14A is a plan view of a rectangular aperture according to the
fourth exemplary embodiment and FIG. 14B is a perspective view
illustrating an electron beam being incident on a sample.

[0038] FIG. 15A illustrates the relationship between a cross section (YZ
cross-section) of the electron beam incident in the longitudinal
direction of the rectangular aperture and reciprocal lattice points and

[0039]FIG. 15B illustrates the relationship between a cross section (XY
cross-section) of the electron beam and the reciprocal lattice points
according to the fourth exemplary embodiment.

[0040] FIG. 16 is a plan view illustrating another rectangular aperture
according to the fourth exemplary embodiment.

[0041]FIG. 17 is a flow chart illustrating a method of evaluating a
distribution of lattice strain on crystal material according to the
fourth exemplary embodiment.

PREFERRED MODES

Exemplary Embodiments

[0042] A summary of an exemplary embodiment of the present disclosure will
be described. For example, by using a scanning transmission electron
microscope as illustrated FIG. 1, if an electron beam is emitted to be
incident on a thin semiconductor device sample as illustrated in FIG. 3A,
a transmitted wave and diffracted waves as illustrated in FIG. 3B can be
observed. As illustrated in FIGS. 5A to 5C and FIGS. 5D to 5F, from
diffracted waves diffracted in different directions, a crystal strain
distribution image corresponding to each of the diffraction directions
can be obtained. In addition, by using a known method such as the NBD
method to quantify strain distribution images, a shear strain
distribution and a principal strain distribution can be grasped from
quantified strain distribution images in a plurality of directions. Thus,
a cause of strain can be determined, and the possibility of occurrence of
a crystal defect can be predicted. The drawings referred to by way of
symbols in this summary are merely used as examples to facilitate
understanding of the present disclosure. Therefore, the present invention
is not limited by the modes illustrated by the drawings.

[0043] Next, each of the exemplary embodiments will be described in detail
with reference to the drawings.

First Exemplary Embodiment

[0044]FIG. 1 is a block diagram illustrating an overall configuration of
a system 10 of evaluating a distribution of lattice strain on crystal
material according to a first exemplary embodiment. The evaluation system
10 in FIG. 1 includes: a scanning transmission electron microscope 100;
and a calculation processing apparatus 200 controlling the electron
microscope 100 and processing measurement data obtained by the electron
microscope 100.

[0045] The electron microscope 100 includes an electron beam source 110
outputting an electron beam used for observing an evaluation sample 300;
an illuminating-system lens apparatus 120 using the electron beam
outputted from the electron beam source 110 to illuminate the evaluation
sample 300; and an imaging-system lens apparatus 150 functioning as an
objective lens focusing an electron beam on a minute spot region of the
evaluation sample 300. An electron beam transmitted by the evaluation
sample 300 or an electron beam diffracted in the forward direction by the
evaluation sample 300 is focused on an electron beam detector 190 by a
projecting-system lens apparatus 160.

[0046] In addition, the electron microscope 100 includes a scanning coil
130 causing the electron beam emitted from the electron beam source 110
to scan the evaluation sample 300. The scanning coil 130 controls the
electron beam to scan the evaluation sample 300. In addition, the
electron microscope 100 includes a deflecting coil 170 selecting a
certain one of the electron beams transmitted by the evaluation sample
300 or diffracted in the forward direction by the evaluation sample 300
and focusing the certain electron beam on the electron beam detector 190.
In addition, the electron microscope 100 includes a scanning coil and
lens control apparatus 180 controlling the scanning coil 130, the
illuminating-system lens apparatus 120, the imaging-system lens apparatus
150, the projecting-system lens apparatus 160, and the deflecting coil
170.

[0047] In addition, the electron microscope 100 includes a sample
orientation control apparatus 140 controlling the orientation of the
evaluation sample 300, to align the crystal axis direction of the
evaluation sample 300 with the electron beam illumination direction. The
sample orientation control apparatus 140 executes fine-tuning of the
direction of the evaluation sample 300, to align the crystal axis
direction of the evaluation sample 300 with the electron beam
illumination direction.

[0048] The sample orientation control apparatus 140 and the scanning coil
and lens control apparatus 180 are connected to the calculation
processing apparatus 200 and are controlled by a control unit 210 of the
calculation processing apparatus 200. The electron beam detector 190 is
also connected to the calculation processing apparatus 200 processing
measurement data detected by the electron beam detector 190.

[0049] The calculation processing apparatus 200 is connected to an input
apparatus 270, a storage apparatus 280, and a display apparatus 290. The
input apparatus 270 includes an operation interface such as a keyboard
and a mouse, so that an operator can control an overall operation of the
evaluation system 10. The storage apparatus 280 can store programs for
controlling measurement data and an overall operation of the evaluation
system 10 and for analyzing measurement data. The display apparatus 290
can display measurement data detected by the electron beam detector 190
as image data and can display results obtained by evaluation and analysis
executed by the calculation processing apparatus 200.

[0050] The calculation processing apparatus 200 includes, as incorporated
functions, the control unit 210 controlling operations of the electron
microscope 100, a strain distribution image extraction unit 221
extracting a strain distribution image from data detected by the electron
beam detector 190, a strain quantification unit 230 using the NBD method
or the like to quantify the magnitude of lattice strain based on a
diffraction image position observed by the electron beam detector 190,
and a stress analysis unit 240 analyzing stress caused on each of the
regions of the evaluation sample 300 based on quantified strain
distribution images in a plurality of directions or the like.

[0051]FIG. 2 is a flow chart illustrating a method of evaluating a
distribution of lattice strain on crystal material. Next, a method of
evaluating lattice strain on a semiconductor device will be described
with reference to FIG. 2. First, in step S1 in FIG. 2, an FIB (Focused
Ion Beam) method or the like is used to form a sectional TEM
(Transmission Electron Microscope) sample of a semiconductor device
having a uniform thickness. In this step, when the sample is formed, it
is desirable that the sample should be adjusted to have a thickness of
200 nm or less and that the sample should not have different structural
distributions in the thickness direction in accordance with dimensions of
the semiconductor device as much as possible (namely, it is desirable
that the sample should be formed to have an uniform structure in the
thickness direction).

[0052]FIG. 3A illustrates an evaluation sample of a semiconductor device.
The semiconductor device in FIG. 3A is a semiconductor device on which a
general silicon-crystal MOSFET is formed. As illustrated in FIG. 3A, the
sectional TEM sample of the semiconductor device is formed so that the
X-axis and Y-axis directions of the single-crystal silicon of an active
region 310 are <110> and <001>, respectively. The active
region 310 is formed in the middle of the semiconductor device in the
X-axis direction, and a shallow trench isolation 320 is formed on either
side of the active region 310 in the X-axis direction. In addition, a
gate 350 is formed on the surface of the semiconductor device (positive
direction in the Y-axis direction), and a contact plug 340 connected to
the active region 310 is formed on either side of the gate 350 in the
X-axis direction.

[0053] Next, the sample formed in step S1 is placed at a position
corresponding to the position of the evaluation sample 300 in the
(scanning transmission) electron microscope 100 of the evaluation system
10 in FIG. 1. In addition, the sample orientation control apparatus 140
is used to control the orientation of the evaluation sample 300 so that
the electron beam is incident on the evaluation sample 300 along the zone
axis thereof (step S2 in FIG. 2). In this example, the zone axis along
which the electron beam is incident is set to be in the Si <110>
direction. In FIG. 3A, the X-axis and the zone axis directions in which
the electron beam is incident are perpendicular to each other. In view of
the crystalline symmetry, both the directions are denoted by <110>.
Namely, in FIG. 3A, the sample orientation control apparatus 140 controls
the orientation of the evaluation sample 300 so that the electron beam is
incident in the direction perpendicular to both of the X-axis and the
Y-axis.

[0054] Next, the deflecting coil 170 is adjusted so that a certain
diffracted wave is captured by the electron beam detector 190 among an
electronic diffraction pattern which is formed downstream of the
evaluation sample 300 (in the traveling direction of the electron beam)
when the electron beam is focused on the evaluation sample 300 (step S3
in FIG. 2). Namely, if the electron beam is emitted to be incident on the
evaluation sample 300 in the direction perpendicular to the X-axis
<110> and the Y-axis <001> in FIG. 3A, a transmitted wave and
a plurality of diffracted waves such as diffracted waves 002, 004, 111,
and 220 are focused near the electron beam detector 190 located
downstream of the evaluation sample 300 (in the traveling direction of
the electron beam), as illustrated in FIG. 3B. The crystal plane of the
active region 310 and the diffracted waves correspond to each other. For
example, in step S3, the deflecting coil 170 is controlled so that the
electron beam detector 190 captures the diffracted wave 220 when lattice
strain in the X direction is evaluated and the diffracted wave 004 or 002
when lattice strain in the Y direction is evaluated. Thus, among the
transmitted wave and the diffracted waves, only a certain diffracted wave
is captured by the electron beam detector 190.

[0055] While the deflecting coil 170 is fixed so that only a certain
diffracted wave is captured by the electron beam detector 190, the
scanning coil 130 is controlled to cause the electron beam to scan the
evaluation sample 300. Consequently, a contrast image corresponding to a
strain distribution in a certain direction (the X <110> direction
or the Y <001> direction in FIG. 3A) can be obtained (in steps S4
and S5 in FIG. 2).

[0056] If no quantitative evaluation is necessary for the lattice strain
(No in step S6), a strain distribution image in each direction is
acquired and the process is ended (step S7). If a quantitative evaluation
is necessary for the lattice strain (Yes in step S6), the strain amount
is quantified by the NBD method or the like and a quantitative strain
distribution image is acquired (in steps S8 and S9). For example, as
illustrated in FIG. 6C, the NBD method can be used to measure a few
points of diffracted wave intensity and lattice strain, and the
measurement values can be processed by linear regression to obtain a
calibration curve. Based on this calibration curve, by displaying a
conversion scale between the diffracted wave intensity and the lattice
strain magnitude as illustrated in FIG. 6B along with a diffracted wave
intensity image as illustrated in FIG. 6A, a quantified strain
distribution in a certain direction can be displayed.

[0057] In FIG. 3A, the TEM sample is formed so that the zone axis in the
electron beam incident direction is set to be in the Si <110>
direction. However, even if the zone axis in the electron beam incident
direction may be set in the Si <100> direction, a strain
distribution based on a diffracted wave can be evaluated. Namely, a
strain distribution in an arbitrary crystal axis direction can be
evaluated.

[0058] Next, a principle of acquiring a contrast image corresponding to
strain in accordance with the above method will be described with
reference to FIGS. 4A and 4B. FIG. 4A illustrates reciprocal lattice
points, an incident electron beam, and an Ewald sphere in a reciprocal
lattice space. Based on Bragg's law, an electron beam incident on crystal
is diffracted from the incident angle by 2θ. The intensity is
determined by an excitation error Sg, which is the distance between a
reciprocal lattice point and the Ewald sphere illustrated in FIG. 4A. The
excitation error Sg is represented by expression 1, assuming that g
represents a reciprocal lattice vector, k: a wave vector, d: lattice
spacing, a: a lattice constant, h,k,l: a lattice plane index, and
λ: an electron beam wavelength.

[0059] If stress is caused on the crystal material and if the lattice
strain is caused, a reciprocal lattice point is moved horizontally as
illustrated in FIG. 4B. Accordingly, if the reciprocal lattice point is
moved, since the excitation error Sg is changed, the diffraction
intensity is also changed. The excitation error Sg when the reciprocal
lattice point is moved can be represented by expression 2.

S g ∝ g + d g 2 k [ expression
2 ] ##EQU00002##

[0060] The electron beam wavelength used by a general electron microscope
is approximately 0.0197 [Å] (when the acceleration voltage is 300
kV). Since the lattice strain evaluated with respect to the Ewald sphere
radius given by the reciprocal of the wavelength is approximately a few %
at most, the relationship between the excitation error Sg and the lattice
strain can suitably be expressed by straight-line approximation. In
addition, while there is an extinction distance ξg as a factor
affecting the diffraction intensity, this is a parameter that is mainly
dependent on the sample thickness and that is changed depending on the
sample thickness or incident intensity. In the present exemplary
embodiment, a sample having a sufficiently uniform thickness can be
formed by the FIB method and the sample can be scanned in a constant
incident direction. Thus, the diffraction intensity is not affected.

[0061] Since a minute device having a complex structure has a complex
lattice strain distribution, use of the NBD method or the CBED method
requires much time for detailed evaluation of a strain distribution.
However, by using the method according to the first exemplary embodiment,
a strain distribution image can be acquired instantly.

Second Exemplary Embodiment

[0062] In the first exemplary embodiment, outputting a quantitative strain
distribution image in each direction is described. In a second exemplary
embodiment, based on the output results according to the first exemplary
embodiment, a principal strain distribution and a shear strain
distribution in an arbitrary direction are outputted to grasp the cause
of crystal strain and to predict the possibility of occurrence of a
crystal defect. FIG. 7 is a flow chart illustrating a process procedure
according to the second exemplary embodiment.

[0063] Before the second exemplary embodiment is described, evaluation
examples of lattice strain caused when a shallow trench isolation (STI)
generally used in a silicon LSI device causes stress to an active region
will be described with reference to FIGS. 8A to 8D. FIG. 8A is a plan
view when an STI interface is formed in alignment with the crystal
orientation in an active region. FIG. 8B is an enlarged view of a portion
near the interface. FIG. 8C is a plan view when an STI interface is
formed with a taper angle with respect to the crystal structure in an
active region. FIG. 8D is an enlarged view of a portion near the
interface.

[0064] As illustrated in FIGS. 8A and 8B, if an STI interface is formed in
alignment with the crystal orientation in an active region, generally,
stress is caused on the crystal structure in the active region in the
directions perpendicular thereto. However, as illustrated in FIGS. 8C and
8D, if an STI interface is formed with a taper angle with respect to the
crystal structure in an active region, the active region is subjected to
not only the strain in the X and Y directions but also shear strain in
the X and Y directions. It is known that such shear strain in a
semiconductor device is a cause of a crystal defect. Thus, it is
necessary that a manufacturing or design process should be controlled so
that such shear strain does not cause a crystal defect.

[0065] However, by evaluating strain only in the X and Y directions, such
shear strain cannot be determined. In addition, since strain in a
direction perpendicular to an STI interface exhibits a maximum level, if
the direction of a principal strain, which is the strain exhibiting a
maximum level, can be determined, a stress source causing the strain can
be determined, making it easier to provide feedback to the semiconductor
manufacturing process. Next, an analysis method according to the second
exemplary embodiment will be described. This method analyzes shear strain
and principal strain, based on lattice strain distribution images in
different directions obtained under different imaging conditions
according to the first exemplary embodiment.

[0066] In the second exemplary embodiment, as in the first exemplary
embodiment, a TEM sample formed by the FIB method is used. Thus, the
stress component in the sample depth direction (in the direction that the
electron beam is transmitted) is set to be zero through stress relaxation
during the FIB processing. Consequently, two-dimensional stress
approximation is possible. Hereinafter, lattice strain in a
two-dimensional stress state will be described.

[0067] In FIG. 3B, strain distributions (εxx), and
(εyy), and (ε.sub.θ, θ=35.3 degrees) in
the X direction, the Y direction, and the 35.3-degree direction can be
obtained based on intensity distribution images obtained by using the
diffracted waves 220, 004, and 111, respectively (step S21 in FIG. 7).
After each of the strain distribution images are acquired, the NBD method
is used to quantify the diffracted wave intensity (step S22 in FIG. 7).
For example, three particular points among the strain distribution images
are measured by the NBD method to execute strain quantification of the
diffraction intensity. As described above, the diffraction intensity and
strain amounts can suitably be expressed by straight-line approximation.

[0068] Next, shear strain is calculated based on the strain distributions
in the three directions (X, Y, and θ (35.3-degree) directions)
(step S23 in FIG. 7). Shear strain γxy can be acquired by
expression 3 in which the X-, Y-, and θ-direction strains are
denoted by εxx, εyy, andε.sub.θ,
respectively.

[0070] By selecting an appropriate direction θ for the strain
ε.sub.θ, the direction exhibiting a strain maximum level
can be determined. Strain in such direction is referred to as principal
strain, and the direction of the principal strain can be determined by
this calculation. Namely, in expression 4, θ exhibiting a maximum
strain amount ε.sub.θ is the direction of the principal
strain, and the strain amount ε.sub.θ in that direction is
the magnitude of the principal strain. For example, as illustrated in
FIG. 9, by representing and outputting the value and the direction of
principal strain at each point as the length and the direction of an
arrow, respectively, a stress source can be determined (step S24 in FIG.
7).

[0071] In addition, the shear strain γ.sub.θ in an arbitrary
direction θ can be obtained by expression 5 (step S25 in FIG. 7).
If silicon /single-crystal is used, the Si (111) plane is a slip plane.
For example, by obtaining a shear strain distribution image on this plane
when θ=54.7 degrees and -54.7 degrees, the possibility of
occurrence of a crystal defect can be predicted.

γ.sub.θ=(εxx-εyy)sin
2θ+γxy cos 2θ [expression 5]

[0072] Namely, according to the second exemplary embodiment, the strain
ε.sub.θ in an arbitrary direction can be obtained by using
quantified strain distributions in a plurality of directions and
expressions 3 and 4. Thus, from the direction exhibiting a maximum strain
level ε.sub.θ, the magnitude and the direction of principal
strain can be obtained. In addition, from the magnitude and the direction
of the principal strain, a stress source can be determined, as
illustrated in FIG. 9.

[0073] In addition, the shear strain γ.sub.θ in an arbitrary
direction θ can be obtained based on expression 5. While the shear
strain in a semiconductor device is a cause of a crystal defect,
according to the second exemplary embodiment, the possibility of
occurrence of a crystal defect can be predicted.

Third Exemplary Embodiment

[0074] The calculation processing apparatus 200 in the evaluation system
10 in FIG. 1 is not necessarily a dedicated calculation processing
apparatus. By causing a general-purpose computer such as an EWS or a PC
to execute a dedicated evaluation program stored in the storage apparatus
280, the general-purpose computer, the electron microscope 100, and
peripheral apparatuses such as the sample orientation control apparatus
140, the scanning coil and lens control apparatus 180, and the electron
beam detector 190 can be allowed to function as the evaluation system 10.
In this case, peripheral apparatuses connectable to the general-purpose
computer can be used as the display apparatus 290, the input apparatus
270, and the storage apparatus 280. In addition, by causing the
general-purpose computer to execute the evaluation program stored in the
storage apparatus 280, the general-purpose computer can be allowed to
function as the calculation processing apparatus 200 including the
control unit 210, the strain distribution image extraction unit 221, the
strain quantification unit 230, and the stress analysis unit 240. Namely,
according to the third exemplary embodiment, by causing a computer to
execute a dedicated program, a scanning transmission electron microscope
and the computer controlling the scanning transmission electron
microscope and processing measurement data obtained by the scanning
transmission electron microscope are allowed to function as the
evaluation system and to execute the evaluation method according to the
first and second exemplary embodiments.

EXAMPLE 1

[0075] Next, example 1 will be described. In example 1, the evaluation
method according to the first exemplary embodiment described with
reference to FIGS. 1 to 4 is applied to a minute device having a trench
gate as disclosed in Patent Document 4, for example. FIGS. 5A and 5B
illustrate a diffracted wave and an intensity image thereof used for
analyzing strain in the X <110> direction, respectively. FIGS. 5D
and 5E illustrate a diffracted wave and an intensity image thereof used
for analyzing strain in the Y <001> direction, respectively. In
example 1, since the lattice strain can be displayed depending on the
strain direction as illustrated in FIGS. 5B and 5E, the strain state can
be grasped easily. In addition, the diffracted wave intensity images as
illustrated in FIGS. 5B and 5E can be displayed by stress analysis
diagrams as illustrated in FIGS. 5C and 5F. FIGS. 5C and 5F are stress
analysis diagrams based on results of a process simulation executed
separately from the measurement in FIGS. 5B and 5E.

[0076] As described above, according to each of the exemplary embodiments
of the present disclosure, the lattice strain can be divided into
directional components to be evaluated instantly, and the strain can be
quantified by using the NBD method. In addition, based on the obtained
results, by using a calculation processing apparatus, a shear strain
distribution and a principal strain distribution, which cannot be
measured by Patent Document 2 or 3 or Non-Patent Document 2, can be
obtained from a strain amount at each point. As a result, the possibility
of occurrence of a crystal defect can be predicted from the shear strain
distribution, and a stress source can be determined by the principal
strain distribution.

Fourth Exemplary Embodiment

[0077] Next, an evaluation system and an evaluation method according to a
fourth exemplary embodiment will be described. In the fourth exemplary
embodiment, based on the principle of the present disclosure described in
the first to third exemplary embodiments and example 1, the strain
direction separation performance (separation of the X-direction strain
and Y-direction strain) is improved.

[0078] As illustrated in FIG. 10A, if stress is caused on the evaluation
sample 300 in a direction perpendicular thereto (in the X direction), an
ideal perpendicular-direction strain is caused. In this case, when the
strain in the X-direction is evaluated, a target reciprocal lattice point
only moves in the X direction, as illustrated in FIG. 10B. However, if
the evaluation sample 300 is an actual LSI device, stress is caused in
complex directions, as illustrated in FIG. 11A. As a result, for example,
complex lattice strain including shear strain is caused. Thus, as
illustrated in FIG. 11B, a target reciprocal lattice point used for
evaluating a strain distribution in the X direction moves not only in the
X direction but also in the Y direction. Consequently, since components
such as the strain in the Y direction and shear strain are superimposed
on a lattice strain distribution image obtained based on a target
diffracted wave, an error is caused when the strain in the X direction is
measured.

[0079] Thus, to improve the strain direction separation performance,
various studies were conducted. As a result, it was found that if the
electron beam incident on the crystal sample (evaluation sample 300) has
greater parallelism, the strain distribution image exhibits a greater
contrast. FIGS. 12A and 12B illustrate shapes of Ewald spheres formed in
the reciprocal space when the electron beam incident on the crystal
sample has a large convergent angle and a small convergent angle (the
illumination wave focused on the crystal sample has greater parallelism),
respectively. As illustrated in FIG. 12A, if the focused electron beam
has a large convergent angle, since electrons are caused to be incident
on the crystal sample in various directions, wider Ewald spheres are
formed in the reciprocal space. As a result, since average excitation
errors are made uniform, the strain contrast is weakened.

[0080] Thus, the fourth exemplary embodiment proposes a rectangular
movable aperture (rectangular aperture 121) as illustrated in FIGS. 13
and 14. FIG. 13 is a block diagram illustrating an overall configuration
of a system 10a for evaluating a distribution of lattice strain on
crystal material according to the fourth exemplary embodiment. Elements
of the evaluation system 10a in FIG. 13 that are different from those of
the evaluation system 10 according to the first exemplary embodiment in
FIG. 1 will be described. In the evaluation system 10a in FIG. 13, an
illuminating-system lens apparatus 120a includes a rectangular aperture
121 (an illumination aperture having a rectangular opening (aperture)).
The rectangular aperture 121 has a rectangular opening (aperture), as
illustrated in a plan view in FIG. 14A. FIG. 14B is a perspective view
illustrating an electron beam being incident on the evaluation sample
300. As illustrated in FIG. 14B, the electron beam emitted from the
electron beam source 110 (see FIG. 13) is narrowed by the rectangular
aperture (illumination aperture) 121 and is then focused on the
evaluation sample 300 by the imaging-system lens apparatus 150 (see FIG.
13). In addition, based on the evaluation system 10a in FIG. 13, a
control unit 210a included in a calculation processing apparatus 200a
controls the illuminating-system lens apparatus 120a including the
rectangular aperture 121.

[0081] By including this rectangular aperture 121 in the
illuminating-system lens apparatus 120a, the electron beam convergent
angles in the X and Y directions can be set to be asymmetrical to each
other. As a result, diffraction intensity change can be obtained,
focusing only the change of a reciprocal lattice point in one direction.

[0082] The fourth exemplary embodiment will be described in more detail
with reference to FIGS. 15A and 15B. FIG. 15A illustrates the
relationship between a cross section of the electron beam in the
longitudinal direction of the rectangular aperture and reciprocal lattice
points (the same as FIG. 12A). FIG. 15B illustrates an X-Y cross section
of a dotted line portion in FIG. 15A (zero-order reciprocal lattice
plane). Since the electron beam in the Y direction has a large convergent
angle, the electron beam is incident in various angles, and wider Ewald
spheres are formed. However, since the electron beam in the X direction
has greater parallelism, no Ewald spheres appears on the zero-order
reciprocal lattice plane. As a result, while small excitation errors are
exhibited in the movement direction of the reciprocal lattice points (in
the Y direction), large excitation errors are exhibited in the other
direction. Namely, since the electron beam has a large convergent angle
in the Y direction, even if, of the focused electron beam, the electron
beam focused in one direction exhibits small excitation errors because of
movement of the reciprocal lattice points in the Y direction, the
electron beam focused in the other direction exhibits large excitation
errors. Therefore, the diffraction intensity is not changed. However,
since the movement in the X direction is due to the electron beam having
greater parallelism, the diffraction intensity is changed. As a result,
it is possible to obtain a lattice strain distribution having reduced
strain errors in the short-side direction of the rectangular aperture.

[0083] In this example, the rectangular aperture 121 is set so that the
long and short side thereof are perpendicular and parallel to the
direction of a certain diffracted wave detected with respect to a
transmitted wave, respectively. For example, in FIG. 3B, when detecting
the diffracted wave 220 diffracted in the X direction with respect to the
transmitted wave, the direction of the rectangular aperture 121 is
adjusted so that the long and short sides thereof are parallel to the
Y-axis and the X-axis, respectively. Similarly, in FIG. 3B, when
detecting the diffracted wave 002 or 004 diffracted in the Y direction
with respect to the transmitted wave, the direction of the rectangular
aperture 121 is adjusted so that the long and short sides thereof are
parallel to the X-axis and Y-axis, respectively. In addition, while a
certain advantageous effect can be obtained as long as the long side is
longer than the short side; however, to reduce strain errors, it is
desirable that the ratio of the long side to the short side be 4:1 or
greater.

[0084] FIG. 16 illustrates a rectangular aperture 121 having a plurality
of rectangular apertures on a single metal plate. This rectangular
aperture 121 in FIG. 16 includes an X-direction evaluation rectangular
aperture 401, a Y-direction evaluation rectangular aperture 402, and a
111-direction evaluation rectangular aperture 403. In this way, strain
distribution images having greater strain separation performance in the
X, Y, and 111 directions can be obtained. Generally, an illuminating lens
system includes a plurality of electron beam lenses, a deflector, and an
aperture. Thus, by using an upstream lens and an aperture, a rectangular
aperture having the long side thereof in an arbitrary direction can be
selected.

[0085]FIG. 17 is a flow chart illustrating a method of evaluating a
distribution of lattice strain on crystal material according to the
fourth exemplary embodiment. When compared with the flow chart in FIG. 2
illustrating the evaluation method according to the first exemplary
embodiment, the flow chart in FIG. 17 includes step S31 before the
detector 190 captures a certain diffracted wave in step S3. In step S31,
the rectangular aperture 121 is adjusted, based on the certain diffracted
wave to be detected. The adjustment of the rectangular aperture 121 (step
S31) may be executed at an arbitrary timing, as long as the adjustment is
executed before the detector 190 captures a certain diffracted wave in
step S3. For example, if the rectangular aperture 121 is a rectangular
aperture including a plurality of apertures (openings) as illustrated in
FIG. 16, when an upstream lens of the illuminating-system lens apparatus
120a is controlled, an aperture matching a certain diffracted wave to be
detected can be selected. Alternatively, the relative direction of the
rectangular aperture 121 and the evaluation sample 300 on the XY plane
may be adjusted by rotating the rectangular aperture 121 and/or the
evaluation sample 300 on the XY plane, for example. Other steps similar
to those according to the first exemplary embodiment are denoted by the
same reference characters, and repetitive descriptions will be omitted.

[0086] Needless to say, the above fourth exemplary embodiment can be
combined with the analysis of a principal strain distribution and a shear
strain distribution in an arbitrary direction according to the second
exemplary embodiment. According to the fourth exemplary embodiment, the
strain direction separation performance can be improved. Thus, it is
expected that a principal strain distribution or a shear strain
distribution in an arbitrary direction can be analyzed more accurately.
In addition, the fourth exemplary embodiment can be implemented by
causing a general-purpose computer such as an EWS or a PC described in
the third exemplary embodiment to execute a dedicated evaluation program.
The computer program caused to function as the evaluation system
according to the fourth exemplary embodiment includes a program for
controlling the relative direction of the opening (aperture) of the
rectangular aperture 121 with respect to the evaluation sample 300 in the
electron microscope 100a.

[0087] According to each of aspects, modes, or exemplary embodiments of
the present disclosure, a strain distribution image is obtained by
selecting a certain diffracted wave. Thus, by evaluating a selected
diffracted wave, a strain distribution image per direction can be
obtained. In addition, the possibility of occurrence of a crystal defect
can be predicted or a stress source can be determined, based on such
strain distribution image per direction. Modifications and adjustments of
the exemplary embodiments and examples are possible within the scope of
the overall disclosure (including the claims and the drawings) of the
present invention and based on the basic technical concept of the present
invention. Various combinations and selections of various disclosed
elements (including the elements in the claims, exemplary embodiments,
drawings, etc.) are possible within the scope of the claims of the
present invention. That is, the present invention of course includes
various variations and modifications that could be made by those skilled
in the art according to the overall disclosure including the claims and
the drawings and based on the technical concept.